The present invention relates to an optical writing apparatus that forms a latent electrostatic image on a photoreceptor by selectively activating a plurality of light-emitting elements in rows for light emission.
FIG. 15 is a schematic diagram of a known copier which forms a latent electrostatic image on the surface of a photoreceptor 2 using an optical writing head 1 in which light-emitting elements are arranged in rows. The photoreceptor 2 is a layer that surrounds the outer surface of a light-sensitive drum 3, which is coupled to a drive unit (not shown) in such a way that the drum is rotated in the direction indicated by arrow 4. Also disposed around the light-sensitive drum 3 are a charge corotron 7, the optical writing head 1, a light concentrating lens system 10, a developer 8, a transfer corotron 9, and a cleaning unit 6.
As the light-sensitive drum 3 is rotated in the direction of arrow 4, a uniform charge layer is formed on the surface of the photoreceptor 2 by means of the charge corotron 7 and the photoreceptor 2 is thereafter illuminated with light from the writing head 1 so as to form a latent electrostatic image. The lens system 10, which concentrates on the photoreceptor 2 light issuing from the plurality of light-emitting elements in the head 1, consists of an array of focusing rod-shaped lenses.
The latent electrostatic image on the photoreceptor 2 is subsequently rendered visible by passage under a developer 8. The resulting toner image on the photoreceptor 2 is transferred to a copy sheet 11 by means of a transfer corotron 9, the sheet 11 being discharged after the toner pattern is fixed by a fixing unit (not shown). The photoreceptor 2 is cleaned of any residual electrostatic image by a cleaning unit 6 and conditioned for another cycle.
The internal structure of the writing head 1 which is used in the manner described above is shown in FIGS. 16 and 17. FIG. 16 is a cross-section of the head, and FIG. 17 is a plan view showing the essential part of the head.
As shown in FIG. 16, a transparent partition 12 is provided on top of an evacuated air-tight case 13 which contains anode electrodes 14. As shown in FIG. 17, each of the anode electrodes 14 is in the form of a tongue which is coated at one end with a phosphor 15 on its top surface. In the following description of the present invention, this phosphor is referred to as a light-emitting element 15.
Cathodes 16 comprising a plurality of filaments are provided beneath the transparent partition 12. When the cathodes 16 are heated by an electric current flowing therethrough, thermions are emitted. If the cathodes 16 are connected to ground and the anode electrodes 14 are supplied with a positive voltage, the emitted thermions will flow toward the anode electrodes 14 and strike the light-emitting elements 15, causing light emission.
As shown in FIG. 17, the anode electrodes 14 are arranged parallel to one another and spaced at equal distances in such a manner that they are partially interleaved with each other. The anode electrodes 14 are electrically insulated from one another and are connected to a drive circuit (not shown) that provides for selective application of a predetermined positive voltage to individual anode electrodes. According to this system, the light-emitting elements 15 are selectively excited for light emission, thereby forming a latent electrostatic image on the surface of the photoreceptor 2. As a result of light emission from one element 15, a single dot of a latent electrostatic image is formed on the surface of the photoreceptor 2. This dot provides a minimum unit of the latent electrostatic image, namely, one pixel of a developed image.
Japanese Unexamined Patent Application Publications Nos.38967/1983, 49148/1984 and 46740/1984 address other various optical writing head configurations.
The known optical writing head 1, discussed above, has various problems. First, the linear arrangement of light-emitting elements 15 requires a certain distance d to be provided between adjacent elements 15, as shown in FIG. 17. The distance d is necessary to ensure reliable electrical insulation between adjacent anode electrodes 14 carrying light-emitting elements 15, and as an inevitable result, a non-light-emitting portion is formed between adjacent light-emitting elements 15. If the optical writing head 1 having such non-light-emitting portions is used to form a latent electrostatic image on the surface of photoreceptor 2, residual charges will be incompletely neutralized in that part of the photoreceptor which faces the non-light-emitting portions. This can be the cause of deterioration of a developed image when the light-emitting elements 15 are seen in the principal scanning direction, or in the direction in which the elements are aligned.
Furthermore, if a mismatch occurs between the speed of rotation of the photoreceptor 2 and the timing of light emission from elements 15, part of the photoreceptor 2 will fail to be illuminated with an adequate amount of light, thus causing deterioration of a developed image when the light-emitting elements 15 are seen in the auxiliary scanning direction, or in the direction in which the elements move, as indicated by arrow 4 in FIG. 5.
Another problem with the previously known optical writing head 1 is that in order to ensure that the individual light-emitting elements 15 can be turned on and off independently of one another, the drive circuit requires as many drive elements and associated drive circuits as light-emitting elements 15. This disadvantageously increases the overall cost of the equipment.